This competitive renewal application addresses dry eye syndromes from the perspective of the new ocularspecific factor 'lacritin1 discovered by us. Lacritin appears to act as a master protector of the ocular surface. Topically applied lacritin triggers elevated and sustained tear flow in rabbits and in a dry eye monkey model. In human corneal epithelial cell cultures, lacritin stimulates MUC16 protein production without affecting MUC16 mRNA. Decreased MUC16 is associated with damage to ocular surface epithelia in dry eye. Lacritin stimulates human corneal epithelial cell proliferation, in keeping with its immunolocalization in basal corneal epithelial cells. Lacritin is also expressed in lacrimal and meibomian glands and in the conjunctiva and is selectively downregulated in blepharitis, the only dry eye disease proteomically examined to date. At micromolar levels, a lacritin fragment is potently bacteriocidal. How does lacritin work? In our initial R01 funding, we discovered that lacritin targets syndecan-1 (SDC1) via a novel heparanase-dependent mechanism. Heparanase is involved in the transit of stem cells in skin and with SDC1 is normally expressed on the ocular surface where SDC1 is anti-inflammatory. SDC1 ligation initiates a cell signaling cascade that is pertussis toxin inhibitable. Since pertussis toxin inactivates the G-proteins Gai or Ga0, the implication is that lacritin also targets a G-protein coupled receptor (GPCR). Using different inhibitor techniques, we then determined that lacritin signals from Ga1, or Ga0 to PLC-PKCa/PLD/mTOR and PLCPKCa/ STIM1/calcineurin/Ca2+/NFATC1 to activate secretory and mitogenic responses. Both pathways feature signaling effectors at the forefront of disease biology apparently not previously examined in the eye (ie, mTOR and NFAT). Our working hypothesis is that lacritin is a multifunctional protector of the ocular surface that acts in novel ways to promote ocular wetting and protect the epithelium against bacteria, wounding and inflammation. Our first aim is to understand specific protein-protein interactions constituting the SDC1-GPCR receptor heterocomplex responsible for epithelial- and ocular-specific targeting by lacritin. Our second aim is to characterize how lacritin biphasic signaling is generated, how one element may promote secretory granule exocytosis, and whether activated NFATC1 and mTOR underlie all lacritin prosecretory activities. Our third aim is to understand lacritin cytoprotective MUC16 and bacteriocidal functions. Specifically, is lacritin relieving microRNA translational repression of MUC16, and if so is this a general ocular surface regulatory mechanism (ie. also affecting aquaporins); and does bacterial infection increase tear lacritin and release lacritin's bacteriocidal fragment?